This invention relates to a fluidized bed heat exchanger and, more particularly, to such a heat exchanger employing a plurality of heat exchange tubes a portion of each of which are immersed in the fluidized bed.
The use of fluidized beds has long been recognized as an attractive means of generating heat. In these arrangements air is passed through a bed of particulate material which includes a mixture of inert material, a fuel material such as high sulfur, bituminous coal and, usually, adsorbent material for the sulfur released as a result of the combustion of the coal. As a result of the air passing through the bed, the bed is fluidized which promotes the combustion of the fuel. The basic advantages of such an arrangement are many and include a relatively high heat transfer rate, a substantially uniform bed temperature, combustion at relatively low temperatures, ease of handling the fuel materials, a reduction in corrosion and boiler fouling and a reduction in boiler size.
When a fluidized bed of the above type is utilized in a heat exchange environment, such as in a steam generator or boiler, a plurality of heat exchange tubes are normally provided for passing water in a heat exchange relation to the fluidized bed. In most of these arrangements, the tubes are disposed in a serpentine relationship with a great majority of the lengths of the tubes extending horizontally, i.e., parallel to the upper surface of the bed. However, this type of arrangement results in less than optimum operational efficiency since, for example, pumps are required to insure satisfactory circulation of the water and steam through the horizontal portions of the tubes. Also, for a given required heat output it is difficult to maintain an optimum temperature range for the combustion and/or other reaction without either mechanically increasing the cooling fluid velocity or increasing the number of tubes and, of course, the number of penetrations through the bed floor or walls. Also, it has been discovered that in the case of horizontal tubes passing a liquid-vapor mixture in the foregoing manner, irregular cooling of the internal surface of the tube occurs which results in hot spotting and premature failing of the tubes. Further, with the use of horizontally disposed heat exchange tubes it is difficult to control the bed temperature in a linear manner.